1. Field of the Invention
This invention relates to off-highway load haul vehicle ride struts and more particularly to a ride strut that uses a compressible liquid and method of maintaining such struts.
Load hauling off-highway vehicles are used in such applications as highway construction and mining. For many years these vehicles were constructed without any spring suspension of any kind except that limited amount of springing provided by the vehicle tires. These unsprung vehicles were operated at relatively slow speeds of the order of 10 miles per hour and at those speeds both the operators and the vehicles were able to tolerate the punishment inflicted by the rigid suspensions.
Modern off-highway haulers no longer are operated at such low speeds. In mining operations, for example, these off-highway vehicles are operated along haul roads at speeds of the order of 40 miles per hour. Carrying capacities, like speeds of operation, have also greatly increased such that payloads may be of the order of 200 tons or more. With such speeds and payloads, sprung suspensions have become a necessity because fatigue of both operator and vehicle would be prohibitive without them.
The first sprung suspensions were rather similar in principle to those used in over-the-highway vehicles. For example, vehicles were constructed with coil spring suspensions and shock absorbers to dampen the spring action.
In recent times, ride struts have been developed which are intended to perform both suspension and shock absorbing functions. These struts have used various types of internally contained springs including rubber, metal and liquids.
While many different ride struts have both been proposed and developed, none has been fully satisfactory because the punishment inflicted in off-highway load hauling causes excessive failures. Failures are inordinantly expensive because (a) struts are expensive, (b) the lost vehicle's production is extremely expensive, and (c) repairs on load haul vehicles are often difficult especially when the breakdowns occur at remote locations where weather conditions may be severe.
So-called liquid spring ride struts have been known for some applications for many years but they have exhibited shortcomings in off highway load haul applications. With these struts, a compressible liquid such as a liquid silicone provides spring suspension. A typical strut has a cylinder that defines a liquid containing chamber. A ported piston is in the chamber and connected to a piston rod. The rod extends from one end of the cylinder. The weight of the vehicle pushes the piston rod into the cylinder to compress the liquid and effect a springing action. The ported piston functions to dampen that action and thus functions as a shock absorber.
One major advantage of a liquid strut is the liquid spring has effectively an endless life so long as the strut itself does not fail. The reason the spring has an effectively endless life is that a liquid strut can be charged with additional liquid to replenish lost liquid. So long as there is a proper quantity of the correct liquid the appropriate spring capacity is maintained, as contrased with metal and rubber springs, spring fatique is eliminated as a factor in suspension life.
Another advantage of liquid struts is that with an appropriately ported piston to permit controlled fluid flow from one side of the piston to the other, the liquid ride strut functions both as a spring suspension and a shock absorber to dampen spring action.
While liquid struts have these and other advantages, the prior struts have also had shortcomings. One shortcoming has been their inability to properly withstand the forces imposed under so-called "rebound" conditions. A rebound condition occurs when a vehicle bounces and leaves the ground or when a wheel drops into a hole or the like such that the strut becomes extended and is in tension supporting the weight of a wheel and axle assembly rather than being compressed to support the vehicle above its wheel and axle.
Attempts have been made to solve the rebound problem in ride struts, but none has been fully satisfactory. For example, attempts have been made to use rubber discs as a rebound cushion but they tend to degrade at an excessive rate when exposed to liquid spring materials such as liquid silicones.
Other attempts at rebound control have not provided both (a) a high dampening rate desired for rebound conditions which will function to inhibit and substantially to prevent damage to the strut or other vehicle components; and (b) ample flow rates through a ported piston to permit the desired rate of piston movement when the strut is operating to function as a vehicle support.